Recommended Citation

Abstract

Changes in the amount of riparian vegetation can vary significantly over large spatial extents, and this variation is influenced by an array of factors. Riparian vegetation is an important component of riverine ecosystems and acts as a filter for terrestrial runoff, a control on bank stability and a key habitat area. Following European settlement in New South Wales (NSW) Australia in the late 18th century, extensive areas of floodplain and riparian vegetation were cleared to allow farming of the rich alluvial soil with close proximity to a water supply. As a result of broad scale vegetation clearing, the extent and connectivity of riparian vegetation suffered. Increased awareness of the role played by a healthy riparian zone resulted in the legislated cessation of riparian vegetation clearing in the 20th and early 21st centuries. To counteract the post-settlement land clearing, the monitoring, revegetation and rehabilitation of these areas has become increasingly important for state and federal governments. This study assessed the changes that have occurred in the levels of riparian vegetation along easterly flowing, coastal draining rivers of NSW over a 30-year period.

This study measured riparian vegetation density and coverage using Landsat Normalised Difference Vegetation Index (NDVI) imagery across 28 river catchments in eastern NSW, assessing 19,750km of channel length overall with the study area representing 2970km2 of riparian zone. The initial and final state images were composed of five-year averages from 1987-1991, and 2009-2015 representing change over two decades. Changes in the spatial extent of riparian vegetation between the two time periods were identified using an image differencing change detection method. GIS software was used to analyse the results of the change detection operation, with various tools implemented to assess the levels of positive and negative change occurring in all major catchments that drain to the eastern seaboard. Gross net change has been assessed in addition to a stratified analysis investigating the change relative to adjacent land use. Utilising a change detection threshold in the NDVI analysis to delineate between significant and non-significant change highlighted spatial patterns across the state. Positive or negative change above the threshold was interpreted as an increase or decrease in vegetation density. Average positive change for the catchments in the study area was 8.6%, negative change was 2.5%, but 89% of the area investigated showed no NDVI change. Further analysis of the change detection results indicated that significant NDVI change was occurring most commonly in or alongside channels bordered by agricultural land. Different climates and land use types across the large spatial extent of the study area create a range of factors driving vegetation change with the study finding variations in streamflow, active conservation, restoration, and rehabilitation of riparian zones being the most influential.